1. Field of the Invention
This invention relates generally to photorefractive materials. More particularly, it relates to photorefractive composites that contain a polydioxaborine.
2. Description of the Related Art
Photorefractive materials are materials that undergo a change in refractive index when illuminated with light. For example, illumination of a photorefractive material with a nonuniform field of light (such as the interference pattern obtained by crossing two coherent light beams) results in the creation of a periodic refractive index modulation (referred to as a grating) inside the material. Photorefractive materials typically display several basic properties: First, such materials are generally capable of photo-charge generation. Illumination of these materials results in the formation of electron/hole pairs (negative and positive charges, respectively). Second, at least one of the charges has mobility in the material. Migration of the most mobile charges from the bright (conducting) regions to the dark (insulating) regions and trapping result in a space-charge field and corresponding periodic electric field. Finally, photorefractive materials have an electro-optic response to the internal space-charge field that gives rise to the refractive index modulation.
A variety of photorefractive materials are known, including inorganic photorefractive materials such as lithium niobate and amorphous photorefractive materials such as photorefractive polymers. Optical devices incorporating photorefractive materials have been fabricated and many have shown promise for applications such as holographic data storage, optical signal amplification, optical switches, and optical correlators. Strong photorefractive responses are typically observed in materials that have high photo-charge generation efficiency, charge transport, and an electro-optical response. The electro-optic response can result from the orientational Kerr effect or from the Pockels effect.
The discovery of a photorefractive effect in organic systems and the subsequent design of organic composites with high diffraction efficiencies has greatly increased the potential market for photorefractive materials. Organic photorefractive materials have been of particular interest because of their structural flexibility and the general ease with which polymers can be fabricated into various shapes suitable for incorporation into devices. However, the photorefractive properties of many of the most promising organic photorefractive material have been found to degrade over time, resulting in relatively short useful lifetimes. Although, in theory, either hole-transport or electron-transport materials can be used in the charge-transport layer in an organic photorefractive composite, in practice only hole-transport materials, particularly polyvinyl carbazole (PVK), have been incorporated into practical devices. Carbazole and other common hole-transport moieties tend to have relatively poor miscibility with polar electro-optic chromophores. The poor miscibility of hole transport materials and electro-optic chromophores is often cited as a cause of the short lifetimes of organic photorefractive materials. Despite extensive efforts to avoid the crystallization or separation of the electro-optic chromophores or to extend the lifetime of hole-transporting organic photorefractive materials, such phase separation remains a significant stumbling block to the commercial development of organic photorefractive materials.
The study of organic electron-transport materials has been limited. The use of an electron-transport moiety as the charge-transport part of a photorefractive material has been reported, see K. Okamata, et al., “Synthesis and Characterization of Photorefractive Polymer Containing Electron-Transport Material,” Chem. Mater., Vol. 11, pp. 3279–3284, (1999). These thioxanthene based organic photorefractive materials were reported to have vanishingly small diffraction efficiencies, long response times and, in most cases, low net two-beam coupling gains. Although these materials may be useful in certain applications, there is a long-felt need in the art for an electron-transport photorefractive article having higher diffraction efficiency, higher two-beam coupling gain and a faster response time in combination with good mechanical, optical, and ease-of-fabrication properties.